Conventionally, some diesel engines have selective reduction catalyst incorporated in an exhaust pipe for flow of exhaust gas, the selective reduction catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen. A required amount of reducing agent is added upstream of the reduction catalyst and is reacted on the catalyst with NOx (nitrogen oxides) in the exhaust gas to thereby reduce a concentration of the discharged NOx.
In a field of industrial plant or the like with flue-gas denitration, it has been well known that ammonia (NH3) is effectively used as reducing agent to reduce and depurate NOx. However, for automobiles, safety in carrying ammonia itself during travel is difficult to ensure, so that in recent years, use of nontoxic urea water as reducing agent has been in practical use. More specifically, if the urea water is added to the exhaust gas upstream of the selective reduction catalyst, the urea water is thermally decomposed in the exhaust gas into ammonia and carbon dioxide gas, and NOx in the exhaust gas is satisfactorily reduced and depurated by ammonia on the catalyst.
On the other hand, for exhaust emission control of a diesel engine, it is insufficient to remove only NOx in the exhaust gas; particulates (particulate matters) in the exhaust gas must be captured through a particulate filter. However, the exhaust gas from the diesel engine in a normal operation state rarely has a chance to obtain a temperature level at which the particulates combust by themselves; an oxidation catalyst having active species such as Pt and Pd is integrally carried by the particulate filter. Specifically, employment of such particulate filter carrying the oxidation catalyst facilitates an oxidation reaction of the captured particulates to lower an ignition temperature, so that the particulates can be removed by combustion even at an exhaust gas temperature lower than ever before.
However, even if such particulate filter is employed, an amount of captured particulates will exceed an amount of treated particulates in operation areas with low exhaust temperature levels. Continued operation with such low exhaust temperature levels may hinder satisfactory regeneration of the particulate filter, resulting in excessive accumulation of the captured particulates in the particulate filter.
Thus, it has been conceived to additionally arrange a flow-through type oxidation catalyst in front of the particulate filter; with accumulation of the particulates becoming increased, fuel is added to the exhaust gas upstream of the oxidation catalyst to forcibly regenerate the particulate filter.
Specifically, the fuel (HC) added upstream of the particulate filter undergoes the oxidation reaction during its passage through the frontward oxidation catalyst. The exhaust gas heated by heat of the reaction and flowing into the particulate filter just behind increases a catalyst bed temperature of the particulate filter to burn off the particulates, thereby regenerating the particulate filter.
However, in a vehicle such as a city shuttle-bus with travel pattern of traveling on congested roads for a long time, the frontward oxidation catalyst hardly has an elevated catalyst bed temperature enough for sufficient catalytic activity and thus an activated oxidation reaction of the added fuel in the oxidation catalyst, failing in effective regeneration of the particulate filter within a short time.
Thus, as shown in FIG. 1, it has been studied to arrange a burner 2 on an entry side of a particulate filter 1 incorporated in an exhaust pipe 10 and burn off captured particulates by combustion with the burner 2 regardless of an operating state of a vehicle, thereby efficiently regenerating the particulate filter 1 within a short time.
In the example illustrated in FIG. 1, the burner 2 includes a fuel injection nozzle 3 for injection of a proper amount of fuel from a fuel tank (not shown) and an ignition plug 4 for ignition of the fuel injected through an injection port of the nozzle. Connected to the burner 2 is a combustion air supply pipe 5 branched downstream from a compressor (not shown) of a turbocharger such that a part of the intake air is guided as combustion air.
Further, arranged downstream of the particulate filter 1 is a selective reduction catalyst 6 with a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. Arranged on an entry side of the selective reduction catalyst 6 is an urea water injector 8 so as to add urea water from an urea water tank (not shown) into an exhaust gas 7. Arranged between an added position of the urea water by the injector 8 and the selective reduction catalyst 6 is a gas mixer 9 so as to facilitate mixing of the urea water with the exhaust gas 7.
In the example illustrated, further arranged between the added position of the urea water by the injector 8 and the particulate filter 1 is an oxidation catalyst 11 which facilitates oxidation reaction of NO in the exhaust gas 7 into NO2. Arranged just behind the selective reduction catalyst 6 is an oxidation catalyst 12 for oxidation reaction of excessive ammonia.
There exist the following Patent Literatures 1 and 2 as prior art document information relating to this kind of technique on exhaust purification catalyst or on heating of exhaust gas by use of a burner.